In a wide variety of applications wherein organic compounds are used or processed, one must be concerned about the occurrence of fouling in the processing equipment. Fouling in an organic stream or system occurs as a result of polymerization or other reaction of at least a portion of the organic components in the stream or system to form a higher molecular weight product having reduced solubility in the organic components. The reduced solubility causes the higher molecular weight product, i.e., the foulant, to separate from the organic components and clog or obstruct transfer lines, settle out from the components, and otherwise coat the surfaces of the processing equipment. The formation of undesirable foulants occurs in process streams having only organic as well as both organic and aqueous phases. The aqueous phase may be merely water entrained in the organic stream during processing, but also includes the water added to quench or cool a reaction or to remove certain water soluble components from the organic stream by a process step, such as steam stripping. Where water is present in the organic stream, the presence of water-soluble dissolved materials which may catalyze or enhance polymerization or other reaction must be considered.
Reaction occurs because the organic compounds are subjected to conditions sufficient to cause modification of the chemical structure of one or more of the organic components of the stream or system. Conditions which affect reactivity include temperature, pressure, pH and presence of trace metals and other contaminants. For example, it is known that in the process of thermally cracking a feedstock blend of naphtha and gas oil to produce short chain thermal cracking products such as ethylene, propylene, ethane, treated pyrolysis gasoline and various mixed hydrocarbon streams, the existing processing temperatures, pressures and presence of trace contaminants cause further reaction of one or more of the thermal cracking products to create oligomers, polymers and oxidized products which are capable of fouling the processing equipment.
The secondary reaction products formed in process streams such as that described above are undesirable for several reasons. First, if the secondary reaction product is soluble in the thermal cracking product stream, it exists as an impurity which must be removed by distillation, solvent extraction, or other separation technique. If alternatively the secondary reaction product is insoluble in the process stream, it tends to settle out of the stream and accumulate in the low-lying portions of the process stream transport system. The insoluble secondary reaction product may also plate out from the stream and coat all exposed walls of the process stream transport system, including piping, pumps, heat exchanger cores, storage tanks, and the like. In either case, the secondary reaction products eventually form substantial deposits within the process stream transport system. These deposits can cause damage to the transport system by building up significant over-pressures within the system, and by limiting the through-put of desirable product. Ultimately, these deposits must be removed, typically by shutting down the entire system and physically removing the deposits. This results in substantial cost, both in lost operating time and in maintenance.
The chemical reactions occurring in organic streams which produce foulants are varied and complex. The most prevalent cause of fouling in an organic stream results from polymerization of one or more organic components of the organic stream. Typically the undesirable foulant polymers are formed by reactions of unsaturated hydrocarbons. Formation of undesirable foulants can be enhanced by the presence of trace organic materials containing hetero atoms such as nitrogen, oxygen, and sulfur.
Polymers are formed in organic streams by free radical chain reactions, which consist of an initiation phase followed by a propagation phase. A free radical is formed from a molecule by the removal of a single electron, the free radical thus having a single odd electron remaining which is available for further reaction. This free radical then reacts with other molecules or free radicals in the organic stream to either propagate the chain or to terminate the chain. The presence of oxygen in the organic stream can itself accelerate the polymerization process by facilitating formation of free radicals. Also, trace amounts of metal impurity carried along from earlier catalytic processes or from the walls of the metal piping itself can act as generators of free radicals. A more detailed explanation of the various reactions involved in the formation of foulants is found in U.S. Pat. No. 4,927,519, issued May 22, 1990, which is incorporated herein by reference.
It is desirable, and highly recommended, to minimize the presence of those materials which cause or enhance formation of foulants, such as oxygen, metals, free radicals and the like. Additional mechanical purification of the organic stream, such as by filtration or centrifugation, aids in reducing the presence of trace metal particles and other insoluble contaminants. Where possible, vacuum and heat are known to be applied to such streams to deaerate or deoxygenate the process stream containing organic materials both with and without water. However, these mechanical treatment methods still leave low levels of contaminants in the stream which subsequently react.
It is known to employ chemical treatments to control fouling deposit formation. U.S. Pat. No. 4,927,519 discloses an antifoulant composition added directly to a hydrocarbonaceous stream comprising a basic antifouling compound wherein one component is selected from the group consisting of alkyl phosphonate phenate sulfide, alkaline earth alkyl phosphonate phenate sulfide, and amine neutralized alkyl phosphonate phenate sulfide, and mixtures thereof, combined with at least one additional compound which is an effective antioxidant, a corrosion-inhibiting compound, or a metal deactivator. U.S. Pat. No. 3,148,225 discloses the use of certain lower alkyl N, N-dialkylhydroxylamines to inhibit popcorn polymer formation during the preparation of synthetic rubber from styrene and butadiene. Notwithstanding the above materials for use in limiting formation of foulants as well as additional known additives having anti-foulant properties, there remains a continued need for alternate and improved methods for inhibiting foulant formation.